In recent years, due to population growth, rapid industrial development, solid waste continuously piled up and dumped onto the soil surface, harmful waste water continuously penetrates into the soil, exhaust emissions from vehicles, the atmosphere of harmful gases and fly ash continuously with the rain landed in the soil. With the improvement of agrochemical level, a large number of chemical fertilizers and pesticides are scattered into the environment, resulting in more and more opportunities for soil to suffer from pollution, and the degree is becoming more and more serious. Under the influence of soil erosion and wind erosion, pollution area continues to expand.
In 2010, nearly 20 million hectares of cultivated land (200,000 square kilometers) are contaminated by organic and heavy metals in China, and this is increasing year by year. The annual area of organic pesticide use is 280 million hectares (2.8 million square kilometers). As a result, the country's agricultural production has already been reduced by more than 13 million tons. Economic losses due to other types of pollution such as pesticides and organic pollutants, radioactive contaminants and pathogenic bacteria, are hard to estimate. Therefore, all substances that interfere with the normal functions of the soil, reduce the yield and quality of crops, and indirectly affect human health through food, vegetables and fruits are called soil pollutants.
When the soil contains too much harmful substances, exceeding the self-purification capacity of the soil, this leads to soil composition, structure and function of the soil changes: microbial activity is inhibited, harmful substances or their decomposition products gradually accumulate in the soil, through the “soil→plant→human body “, or through the” soil→water→human body” indirectly absorbed by the body to reach the extent of endangering human health, that is, soil pollution.
A variety of contaminated soil, relates to soil heavy metal pollution, soil organic pollution, soil pollution, soil microbial contamination, soil acid rain pollution.
The heavy metal pollution in soils mainly includes elements of significant biological toxicity such as mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr) and arsenic (As), Copper (Cu), nickel (Ni) and other elements; mainly from pesticides, wastewater, sludge and atmospheric deposition. For example, mercury is mainly from mercury-containing wastewater; cadmium and lead are mainly from smelting emissions and vehicle emissions. Arsenic (As) is widely used as insecticides, fungicides, rodenticides and herbicides.
Soil organic pollution mainly includes more than 120 varieties of pesticides. Organic pesticides according to their chemical properties can be divided into organic chlorine pesticides, organic phosphorus pesticides, carbamates and phenoxy alkanoates pesticides. The first two types of pesticides are extremely toxic, and organic chlorine pesticides are not easily degraded in the soil, causing heavy pollution to the soil. Although organophosphate pesticides are easily degraded in the soil, they are also widely contaminated due to heavy use. China's average per hectare of farmland pesticide application is 13.9 kg, about 1 times higher than in developed countries, the utilization rate is less than 30%, causing a large area of soil pollution.
The disadvantages of soil chemical fertilizer pollution includes: increasing soil heavy metals and toxic elements, promoting soil acidification, leading to nutrient imbalances and nitrate accumulation, soil structure destruction, and microbial activity reduction, etc. This causes a series of problems to the sustainable development of agriculture. To increase production, the application of fertilizers is an important choice for most farmers. Therefore, our country has become the country that has the largest number of chemical fertilizers in the world.
Soil pathogenic microbial contamination includes: pathogens, bacteria and viruses; excrements from humans and animals and sewage for irrigation (untreated domestic sewage, especially hospital sewage), etc. If people directly contact the soil containing pathogenic microorganisms, this may have an impact on health. If eating vegetables, fruits contaminated by soil, this means indirect contamination.
Soil acid rain pollution mainly includes: industrial emissions of SO2, NO and other harmful gases react in the atmosphere to form acid rain, natural precipitation into the soil, causing soil acidification.
Soil remediation: refers to the use of physical, chemical or biological methods to fix, transfer, absorption, degradation or conversion of contaminated soil in the site, to reduce its content to an acceptable level, or to convert toxic and harmful pollutants into harmless substances. The repair method is divided into two kinds of methods: repairing in situ and ectopic repair. Ectopic repair involves excavation and soil transport, this not only will undermine the original soil structure, there will be secondary pollution problems.
At present, the technology to repair contaminated soil include the following:
Phytoremediation method. This Is recognized as an ideal soil treatment in situ technology, but there are obvious shortcomings: short plants, low biomass, slow growth and long growth cycle; phytoremediation is also subject to environmental conditions such as soil type, temperature, humidity and nutrition. Phytoremediation takes three to five years, meaning that the time it takes for the soil to expire is three to five years, affecting economic efficiency. In addition, the cost of phytoremediation is high, reaching about 200 Chinese Yuan per cubic meter of soil, and the cost of remediation per mu is as high as 80,000 to 100,000 Chinese Yuan. The restoration cost per hectare of soil reaches 120 million to 150 million Yuan. The high cost is simply unbearable to the local government and peasants.
Chemical leaching method. This means use of chemical agents such as surfactants, pickling agents, so that the heavy metals in the soil after washing out the wastewater treatment. This method is not only very costly and far higher than phytoremediation, but also brings new pollutants, such as surfactants and pickling agents, into the soil, and the physical structure of the soil can be greatly affected. Become a new source of pollution.
Guest soil method. The contaminated soil is buried more than a meter deep in original place or dug up and transported away, and then backfilled with clean soil. For the remediation of large-scale contaminated soil, not only a large amount of manpower, material and financial resources are needed (repairing acres of contaminated land leads to millions of dollars), but also the source of clean soil is hard to guarantee. In addition, the guest soil method easily destroys the soil structure, leading to soil fertility and micro-ecological environment decline, and the transfer of the contaminated soil can easily lead to the release of organic and heavy metals, the pollution is transferred into the groundwater, resulting in new groundwater pollution.
Microbial leaching method. Organic matter and heavy metals are washed out by adding a microbial flora to the soil, wherein the microbial flora can collect the organic matter and heavy metals. This method is not only high in cost but also consumes a large amount of microbial strain, which is difficult to operate due to the difficulty of collecting microbial eluent for a large area of soil pollution.
Electrochemical method. In the soil, through the action of current, water-soluble heavy metal ions are enriched to the cathode surface. At present the technology is only in the laboratory to explore. Due to the distance of the current effect is too small, and the area of polluted land is too large, this technology is difficult to implement in engineering, and regardless of its high operating costs unbearable.
In summary, how to develop practical and applied to a variety of contaminated soil comprehensive repair methods and techniques have become the future of soil pollution control new problems.